A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICS). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticule, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. Including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, high-precision motion control systems may be found for performing the stepping and scanning operations. In such a motion control system, motion controllers are used to control actuators which, e.g., move a mask table or reticule stage RS, or a substrate table or wafer stage WS in several degrees of freedom.
An objective in designing such motion controllers is to guarantee robustness of the associated motion system, and to obtain a high bandwidth. In particular, it is desirable to guarantee robustness for variations of the system which arise from position dependent dynamics, from variations of mechanical properties of materials used, such as stiffness or damping properties, and from the mass of the object moved, which may be influenced by different reticles or wafers having different masses and being connected thereto. As an example, it is desirable that the motion controller be robust for all positions of the object moved, taking into account that the point on an object, such as a support structure or substrate table, where a position thereof is measured changes during a scan, e.g. as a result of mechanical eigen frequencies of the object.
However, since the motion controller should be stable for all of these variations in different positions of the object, the bandwidth of the motion system may be limited. A limited bandwidth of the motion control system limits the settling time and the throughput of the lithographic apparatus.